Acid-Base Equilibrium

acid

acid

ARRHENIUS DEFINITIONS:

Substances that ionize in water to produce hydrogen ions, H+(aq).

base

ARRHENIUS DEFINITIONS:

Substances that dissociate in water to produce hydroxide ions, OH-(aq).

acid

BRONSTED-LOWRY DEFINITIONS:

The proton, H+(aq), donor in a reaction.

• e.g. hydrofluoric acid + water

base

BRONSTED-LOWRY DEFINITIONS:

The proton, H+(aq), acceptor in a reaction.

• ammonia + water

amphoteric substance

Substances that can act as either an acid or a base.

• Water acts as a base with hydrofluoric acid, but as an acid with ammonia.

strong acid

Completely ionizes in water.

weak acid

Only a fraction of the molecules ionize in water.

conjugate acid-base pair

Molecules or ions related by the transfer of a proton. An acid becomes a conjugate base when a proton is removed, and a base becomes a conjugate acid when a proton added.

ion-product constant

percent ionization

The ratio of the concentration of ionized molecules at equilibrium, to the initial concentration of the weak acid, expressed as a percent:

• A strong acid almost completely ionizes in water to form hydrogen ions, and its % ionization > 99%.

• A weak acid partially ionizes in water to form hydrogen ions, and most have a % ionization < 50%.

acid ionization constant

Ka

A weak acid partially ionizes, donating protons in water. At equilibrium, there will be BOTH the weak acid, HA, and its conjugate base, A-, in the solution.

monoprotic acid

Has one ionizable hydrogen atom.

• e.g. HCl (aq), HNO3 (aq), HClO (aq)

polyprotic acid

Has more than one ionizable hydrogen atom.

• e.g. H2SO4 (aq), H3PO4 (aq)

With the exception of sulfuric acid, all of these acids are weak. They do not ionize completely in one step, but rather in two or more steps. Each step has an ionization constant: Ka1, Ka2, Ka3, etc.

base ionization constant

Kb

A weak base partially ionizes, accepting protons in water. There are two main categories:

1. Neutral substances with an atom that has a nonbonding pair of electrons that can accept H+ (e.g. ammonia and the amines).

2. Anions of weak acids (conjugate base).

At equilibrium, there will be BOTH the weak base, B, and its conjugate acid, BH+, in solution.

weak base

Partially ionizes, accepting protons in water.

salt

An ionic compound that forms during a neutralization reaction between an acid and a base. The cation (positive ion) comes from the base, and the anion (negative ion) comes from the acid.

hydrolysis

A reaction in which one of the ions from a salt reacts with water, forming either an acidic or basic solution.

Different salts will produce various pH ranges in an aqueous solution depending on how the ions of the salt react with water to produce either H3O+ or OH- ions.

We can use the strengths of the “parent” acid and base to help us predict the pH of the salt solution.

We need to look at the salt and ask:

1. Which “parent” acid and base reacted to form the salt?

2. If the “parent” acid/base strong or weak?

• *If it is a strong acid/base: fully ionize in water, salt ion will NOT hydrolyze with water (acts as pH-neutral)

• *If it is a weak acid/base: partially ionize in water, salt ion WILL hydrolyze with water (acts as weak base/acid)

titration

Used to determine the molar mass, concentration, or pH of one of the compounds in a neutralization reaction.

equivalence point

Point in a titration when the acid and base that are present completely react with each other (n(a) = n(b)).

• Located in the middle of steep rise of titration curve.

endpoint

Point in titration where indicator changes colour.

• pH = pKa of the indicator (logKa)

• Pick an indicator that changes colour close to the equivalence point so there is negligible volume difference.